1. Field of the Invention
The present invention relates to a building intercom system with a short circuit detection and correction system.
2. Description of the Prior Art
It has been an increasing concern by apartment builders to provide an apartment intercom/door release system that not only is secure, but also reliable. As apartment buildings with 200 apartments or more become commonplace, it is extremely burdensome and time consuming to trace every wire in an intercom system to locate a fault. In addition, the entire intercom system may be out of order until the fault is found.
Systems have been developed in the past to provide an intercom system with increased reliability and/or ease of use.
U.S. Pat. No. 3,484,561 issued to Matthews on Dec. 16, 1969 shows a general intercom system which uses a standard telephone in place of a wall unit in each tenant apartment. The system includes a call waiting tone to alert a tenant that a visitor is trying to reach the tenant over the intercom system.
U.S. Pat. No. 3,728,493 issued to Ter Veen on Apr. 17, 1973 shows an intercom system which has an entrance phone with a two way speaker, and uses a normal, "standby" condition to minimize the amount of energy required by the system.
U.S. Pat. No. 4,658,095 issued to Santiago on Apr. 14, 1987 shows a two wire intercom system with a standby mode to reduce energy requirements of the system. A timer is disclosed for shutting down the intercom system if a tenant does not answer within 20-30 seconds. Also, the intercom system is disclosed to continue to operate in the presence of a short circuit, but no further details are disclosed.
U.S. Pat. No. 5,321,742 issued to Stevens on Jun. 14, 1994 shows an intercom system which uses a touchtone telephone in each apartment, each telephone being connected to a door speakerphone. Activation of a keypad on the speakerphone sends a "paging" signal to a touchtone telephone of a selected apartment.
Several developments have also been made to help reduce the amount of time required to track down and locate short circuits in large scale electrical systems.
U.S. Pat. No. 3,989,908 issued to Budrys et al. on Nov. 2, 1976 shows a circuit diagram for a public address system which uses a line-end resistor to detect a change in impedance in the line caused by a short circuit.
U.S. Pat. No. 4,884,034 issued to Guzman on Nov. 28, 1989 shows a current probe used in an underground power distribution system. The probe is used to detect short circuits or faults in the underground distribution system.
Japanese Patent 61-236261 published on Oct. 21, 1986 shows a data monitoring system for a private exchange which records and compares data on the exchange bus immediately prior to the fault to help determine the cause of the fault.
Japanese Patent 63-69362 published on Mar. 29, 1988 shows a circuit for interrupting a call after a preset time period to protect the system during short circuiting of a related corridor light system.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.